Malaria Transmission by Anopheles subpictus (Diptera: Culicidae) in a New Irrigation Project in Sri Lanka

Abstract

Indoor resting Anopheles subpictus in a new irrigation scheme in Sri Lanka were investigated during 1989-1990 for malaria infection by dissection and ELISA, and human feeding rates by ELISA analysis of blood meals. Indoor resting abundance was 22.3 females per house per 15-min catch in April-June 1989, 2.2 in November-December 1989, and 7.5 in April-June 1990. ELISA-based malaria infection rates (Plasmodium vivax and P. falciparum combined) were 1.4% during April-June and 3.2% during November-December 1989, and 12.5% during April-June 1990. Dissection and ELISA confirmed the presence of oocysts and sporozoites of P. falciparum in 1990. Human blood was detected in 4.3, 0.8, and 5.2% of analyzed meals, respectively, during the three sampling periods. Estimates of the entomological inoculation rate (EIR) ranged from 0.00006 to 0.005 in different samples and vectorial capacity (VC) was 0.0005 for the 1990 sample. Thus, An. subpictus is incriminated as a vector of human malaria in Sri Lanka.

Malaria vectors in a traditional dry zone village in Sri Lanka.

“…Serologic evidence for VK247 infection in humans in Sri Lanka has also been reported. 21 It is evident that previous ELISA studies [4][5][6][7][8] that used only the PV-210 polymorph would have underestimated the CS protein rates of P. vivax in Sri Lankan anophelines and thereby the VC and EIR estimates based on these rates.…”

“…Details of the methodologies used have been published previously. 4,5,12 Both HBC and CBC were restricted to the first half of the night due to hazards associated with the study area being located close to a civil war zone within Sri Lanka. However, this limitation would not have significantly biased anopheline catches, since previous studies in Sri Lanka have shown that 65-85% of the host-seeking females of the species collected herein bite before midnight.…”

“…2,3 However, recent field studies have produced evidence of the involvement of several other anopheline species in malaria transmission, based on ELISAs to detect the circumsporozoite protein (CS) of Plasmodium vivax and P. falciparum, the only malaria parasite species that are currently known to be present in Sri Lanka. [4][5][6][7][8] This has led to some confusion regarding the true field vector status of these different anopheline species that have been incriminated by ELISA. The overall relative contribution of different local anopheline species to transmission has been quantified, 7 but there have been no studies that link temporal trends in vector population and transmission dynamics to trends in human malaria to clarify which species are of real significance in initiating and maintaining transmission.…”

“…Studies on epidemiologic entomology relating to malaria in Sri Lanka have been concentrated in rural habitats associated with modern irrigation development projects such as the Mahaweli Project, 4,5,8 or in village habitats in the southern part of the island. 6,7 Vector bionomics and transmission parameters have not been studied in traditional tank-based villages that still form a substantial small-scale irrigation network in the rice-producing low country dry zone of Sri Lanka.…”

Long-lasting transition toward sustainable elimination of desert malaria under irrigation development

“…By increasing surface water levels, irrigation modifies ecohydrological conditions of the landscape, creating more standing bodies of water for longer periods of time (5), thereby increasing the abundance of mosquito breeding sites and adult vector populations (6)(7)(8)(9)(10)(11)(12)(13). In addition, agricultural development can increase the frequency of human-vector contact, when human labor and mosquito breeding seasons are synchronized (14), and promotes migration to newly irrigated areas (15), thus changing the spatial scale of malaria transmission.…”